1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, which are suitable for an image pickup apparatus using a solid-state image pickup element, such as a video camera, an electronic still camera, a broadcasting camera, or a monitor camera, or an image pickup apparatus such as a silver-halide film camera.
2. Description of the Related Art
In recent years, an image pickup apparatus using a solid-state image pickup element, such as a video camera, a digital still camera, a broadcasting camera, or a monitor camera, as well as a silver-halide film camera is downsized as the entire apparatus. Then, an image taking optical system used for the image pickup apparatus is required to be a high-resolution zoom lens having a short total lens length, a compact (small) size, and a high zoom ratio (high magnification ratio). As a zoom lens which meets the requirements, there is known a positive-lead type zoom lens having a lens unit including, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having the positive refractive power, and a rear group including at least one lens unit following those units. As the positive-lead type zoom lens, there is known a zoom lens constituted of four lens units having positive, negative, positive, and positive refractive powers, in order from the object side to the image side.
In U.S. Pat. No. 7,738,181, a low-dispersion material is used for a positive lens of a first lens unit, so as to perform appropriate correction of chromatic aberration at a telephoto side. In addition, there is known a zoom lens constituted of five lens units having, in order from an object side to an image side, positive, negative, positive, positive, and positive refractive powers (U.S. Pat. No. 7,286,304). In addition, there is known a zoom lens constituted of five lens units including, in order from an object side to an image side, positive, negative, positive, negative, and positive refractive power. U.S. Pat. No. 7,177,092 discloses a zoom lens having a zoom ratio of approximately 10.
In general, in order to obtain a zoom lens having a downsized entire system while having a predetermined zoom ratio, a refractive power (optical power=reciprocal of a focal length) of each lens unit constituting the zoom lens needs to be enhanced to reduce the number of lenses. However, such a zoom lens has large aberration variation accompanying zooming, and it is difficult to obtain high optical performance over the entire zoom range. In particular, the effective front lens diameter becomes large and downsizing of the entire lens system becomes insufficient. At the same time, it is difficult to correct various aberrations such as chromatic aberration at a telephoto end.
In order to obtain a high zoom ratio and downsize the entire lens system while obtaining good optical performance in the four-unit zoom lens or the five-unit zoom lens described above, it is important to appropriately set refractive power and lens configuration of each lens unit and a move condition of each lens unit in zooming. In particular, it is important to appropriately set lens configurations of the first and second lens units and move conditions of the first and third lens units in zooming. Unless those structures are set appropriately, it is difficult to obtain a zoom lens having a small entire system, a high zoom ratio, and high optical performance.